1. Field of the Invention
This invention relates to systems for enhancing the capability of a pilot to successfully operate an aircraft such as an attack helicopter in a low light level environment and, more particularly, to systems providing helmet mounted displays which combine infrared or other image detection and instrumentation symbology in a biocular helmet mounted display.
2. Description of the Prior Art
Various systems are known in the prior art which have been developed to display symbology information or reticle information to an eyepiece for viewing by a pilot. Images from sensors such as radar, forward looking infrared (FLIR), low-light-level television imagery, electroptical sensors and the like are combined with such information and presented to the pilot eyepiece. A number of such systems are described in an article entitled "Helmet-Mounted Display Interest Revives", Aviation Week & Space Technology, Feb. 24, 1969. Some of these systems rely on binocular type arrangements with image intensifiers for amplifying the light level in the scene viewed by the pilot. Such systems suffer from the disadvantage that the field of view presented to both eyes may not be the same, the image intensifier arrangement is cumbersome and heavy, and the eyepiece(s) constitute an object in front of the pilot's face which is likely to cause injury in the event of impact.
Other systems described in the article incorporate one or a pair of cathode ray tubes which generate images projected through optics for viewing by the pilot. The cathode ray tubes are mounted along the sides of the helmets and may provide the pilot with only a monocular view. In some of these versions, the pilot is required to view the display with one eye through an eyepiece equipped with a rubber cup to shut out ambient light. Variations in light intensity in the scenes presented to the pilot's eyes tend to produce eyestrain and the rubber cup becomes uncomfortable, possibly inducing headaches, in extended use. Furthermore, the viewing of separate images by the two eyes requires substantial training for a pilot to adapt to such an image presentation and tends to be tiring and confusing. Potential confusion or rivalry between displayed information and external background is to be avoided, if possible. The use of partially reflecting mirrors to display the images in front of the pilot's eyes undesirably cuts down on the available light for the outside scene which the pilot views directly. Also the distribution of components of a helmet mounted display is an important consideration, since the moments of inertia with respect to the wearer's spine should be minimized in order to avoid wearer discomfort and fatigue.
A system called "Nighthelm" of Marconi Avionics Limited has been developed for use in modern attack helicopters which employ a steerable FLIR sensor to provide night flying imagery together with target sensing capability. The pilot can view the aircraft instruments and the real world through a pair of eyepieces on which are also projected images developed from a pair of image intensifiers situated along the helmet above the eyepiece. By switching to another mode, the pilot may view on one eyepiece an image derived from a cathode ray tube (CRT) which displays FLIR imagery from the remote steerable sensor system. This system does not present images from the CRT to both eyes and thus, in the CRT viewing mode, presents different images to the pilot's eyes. Furthermore, the eyepieces are of heavy thick plastic, and they and the binocular/image intensifier portion of the system present a risk of injury to the pilot upon impact. A further disadvantage of a binocular system is the requirement that the binoculars be adjustable to accommodate the interpupillary dimensions for various users. Thus this system is also subject to many of the problems and disadvantages discussed above.
Withrington U.S. Pat. No. 3,940,204 discloses the use of holograms in head-up and in helmet mounted displays. The holographic lens of those displays is particularly constructed to correct particular optical aberrations and further compensation is afforded by the use of a relay lens having cylindrical surfaces. The image presented at the visor is monocular.